Nitrile rubbers, often also abbreviated to “NBR”, are understood to mean rubbers which are co- or terpolymers of at least one α,β-unsaturated nitrile, at least one conjugated diene and optionally one or more further copolymerizable monomers. Hydrogenated nitrile rubbers (“HNBR”) are understood to mean corresponding co- or terpolymers in which all or some of the C═C double bonds of the copolymerized diene units have been hydrogenated.
For many years, both NBR and HNBR have occupied an established position in the specialty elastomers sector. They possess an excellent profile of properties in the form of excellent oil resistance, good heat stability, excellent resistance to ozone and chemicals, the latter being even more pronounced its the case of HNBR than in the case of NBR, NBR and HNBR also have very good mechanical and performance properties. For this reason, they are widely used in a wide variety of different fields of use, and are used, for example, for production of gaskets, hoses, belts and damping elements in the automotive sector, and also for stators, well seals and valve seals in the oil production sector, and also for numerous parts in the electrical industry, mechanical engineering and shipbuilding. A multitude of different types are commercially available, and these feature, according to the application sector, different monomers, molecular weights, polydispersities and mechanical and physical properties. As well as the standard types, there is increasing demand particularly for specialty types featuring contents of specific termonomers or particular functionalizations.
The market for carboxylated nitrile rubber (also referred to in abbreviated form as “XNBR”), a terpolymer based on at least one α,β-unsaturated nitrile, at least one conjugated diene and at least one monomer containing carboxyl groups, in the case of industrial rubber articles is traditionally in the fields of industrial drive technology, conveying technology, the textile industry, seals in the automotive and industrial sectors, and other specialty applications.
The particular properties of XNBR, such as                very low abrasion and good wear resistance,        excellent vulcanizate properties in relation to strength and stress values,        excellent binding to polar substrates as a result of a possible reaction of the substrate with the carboxyl group of the termonomer and        hydrophilicity, which is likewise attributable to the repeating units of the termonomer containing carboxyl groups,have for many years enabled coverage of some important fields of use by XNBR.        
However, there are limits to wider diversification, firstly resulting from the higher raw material cost of XNBR compared to NBR, and secondly resulting from the hitherto unavoidable use of a crosslinking system composed of metal oxide and a standard sulphur system, in order to obtain vulcanizates with a useable profile of properties. Metal oxides such as, more particularly, zinc oxide, however, are environmentally toxic substances and therefore undesirable in principle.
The crosslinking system composed of metal oxide and sulphur or a sulphur donor enables the vulcanization involving the carboxyl groups in the termonomer repeating units and the double bond in the polymer chain, but                processing reliability often does not meet the demands of modern operation, and the handling of the vulcanizable mixture is difficult and may be associated with additional costs, and        thermal stability, especially in relation to compression set properties, and resistance to heat ageing are much lower than in the case of nitrite rubber, particularly at high temperatures, because of the metal oxide used.        
The preferred use of a combination of zinc oxide and sulphur as a crosslinking system for XNBR is described in Rubber Chemistry and Technology 30 (1957), 1347. A reaction, referred to in Macromolecules, Vol. 3, No. 2, 147 (1970) as “cluster-like” crosslinking, of the dispersed zinc oxide particles with the carboxyl groups of the repeat termonomer units in the XNBR was recognized as the essential cause of many of the excellent properties mentioned, but also leads to the abovementioned problems.
In Kautschuk, Gummi, Kunststoffe 53, 415 (2000), it is stated that a certain improvement in relation to processing is enabled through use of zinc peroxide rather than zinc oxide. However, this variant does not constitute a satisfactory solution overall because of the handling and availability problems associated with this product, and the persistent weakness in relation to the vulcanizate properties.
There has therefore been a search for solutions to the problem which do not need metal oxide as part of the crosslinking system.
There have been different approaches to this in the past. Examples include crosslinking systems based on diamines, diepoxides (diglycidyl ethers of bisphenol A), as used in the manufacture of epoxy resins, carbodiimides, blocked isocyanates and, as described in Journal of Applied Polymer Science, 80, 1925 (2001), thiophosphoryl polysulphides. However, all these processes work in one way, either through reaction with the carboxyl group of the termonomer or through reaction with the double bond of the conjugated diene monomer, show disadvantages of various kinds, and have therefore remained without any great significance in industrial practice.
As detailed in GAK August 2007, volume 60, p. 494ff. by D. Schneegans, R. Gattringer and R. Bauer, only the use of peroxides in “2K technology” has gained a certain significance.
The book “Vulkanisation & Vulkanisationshilfsmittel” [Vulcanization & Vulcanization Aids] by Werner Hofmann, as early as in the 1965 edition, mentioned resin crosslinking for carboxylated nitrile rubbers. There are no known further developments and publications with the aim of introducing this resin crosslinking into industrial practice.
It was thus an object of the present invention to provide vulcanizable mixtures based on nitrile rubbers containing carboxyl groups, which have sufficient processing reliability in the course of processing, are thus easily to handle, and additionally have high thermal stability, especially in relation to compression set properties and resistance to heat ageing.